Offsetting

[Richard]

Hi everyone, hope you are all ok.
Does anyone know of a proper definition of offsetting on a grid via a baseline?
Thanks.

[1man1desk]

Very simple.

An offset is a horizontal measurement taken at 90 degrees from the baseline.

With reference to the 'graduate consultants' thread - this answer was provided by our graduate.

1man1desk

to let, fully furnished

[BAJR Host]

the two measurements you need are...
1. distance along tape from base point.
2. distance from tape at 90degs to point to be measured.

that said... ofsetting is only useful for a few points, should not be used over long distances... and is painfully slow compared to good old planning.

I once had to help plan an Assyrian courtyard using offset.... wept ... went off to the market... had a planning frame built and then brought a look of joy and disbelief to team I was helping... what was this magic

Offset fine for a few measurements... bad for a good site plan.

Another day another WSI?

[kevin wooldridge]

On a traditional archaeological site the grid used for planning is formed from a series of squares or ocassionally rectangles. Within that grid archaeological plans can be created by the use of a planning frame or by measuring the outlines of features from tapes stretched along the gridlines.

Archaeological sites however are rarely entirely square or rectangular and there will always be areas that need planning that fall outside of the mainsite grid and/or into which a planning frame cant be fitted. Planning in these areas is normally achieved by measurements [u]offset</u> from the main site grid. So offsetting is therefore a process, not a measurement in its own right.

Sometimes us archies will plan an archaeological site from a single baseline rather than a site grid, in which case each and every measurement is [u]offset</u> from the baseline.

Is this for an essay, by the way?

[muddyandcold]

Know a husband & wife team in the south-east (and a RAO) who regularly rely upon off-setting to locate trenches over long distances and with old and stretched tapes......

[the invisible man]

Way back before most of you were born, in the early 70's, there were no Total Stations, EDM's, no GIS or even Autocad. Commercial survey was basically all done by offsets. You will all have done basic survey though. The principle was exactly the same but without batteries. Stations are surveyed in by theodolite around a field and offsets taken from baselines between them in the form of a chain (yes a real one) or a 100m tape, to the boundaries (usually in a large and very prickly hedge).

The baselines were long but the offsets were short. I recall that the essential equipment list included a football.

We owe the dead nothing but the truth.

[achingknees]

quote:Originally posted by the invisible man

Way back before most of you were born, in the early 70's, there were no Total Stations, EDM's, no GIS or even Autocad. Commercial survey was basically all done by offsets. You will all have done basic survey though. The principle was exactly the same but without batteries. Stations are surveyed in by theodolite around a field and offsets taken from baselines between them in the form of a chain (yes a real one) or a 100m tape, to the boundaries (usually in a large and very prickly hedge).

The baselines were long but the offsets were short. I recall that the essential equipment list included a football.

We owe the dead nothing but the truth.

I still regularly triangulate from a baseline to avoid the inaccuracies of plain right angle off-setting. A survey point is measured from either end of the baseline. Using a pair of compasses you then intersect two arcs. Very useful if the EDM hasn't had its battery charged.

[1man1desk]

The biggest problems with both offsetting and triangulation are:

Topography - if you site is not flat, you get all sorts of distortion on the slope;

Calibration - tapes do stretch, so you have to keep them well calibrated and replace them when knackered; often not done in archaeological units (at least in the old days when I worked there);

Sagging - if the tape has to be held in the air, unsupported by the ground, you get more inaccuracy from sagging. You need to keep them stretched tight, but that can make it hard to hold the 0 end steady and also knackers the calibration.

Human error - extremely easy to get your right-angles wrong, plus a whole host of other easy mistakes to make in what is basically a simple process.

So, the basic lessons are:

- only use hand-measurement as your principal method over short distances, or to cross-check digital surveying for errors;

- make sure your equipment is in good condition;

- don't rush, and make sure the results are checked by the supervisor.

1man1desk

to let, fully furnished

[the invisible man]

Another, and I suggest perhaps the most important lesson, is to use the technique that is most appropriate for the particular task and conditions. You could not triangulate every "offset" along a baseline when surveying my large field, for example: you'd be there for months and that is why the standard survey book looks like it does - noting down the offset dimension is known, imaginatively, as "booking". You may well triangulate to a certain prominent feature, perhaps.

I would also stress that I am not referring to archaeoelogical recording, I am talking about site surveys relating to construction and development.

We owe the dead nothing but the truth.

[muddyandcold]

Good advice one man. Can i add if you must use triangulation, always try and use a steel tape as this eliminates tape stetch...